The long-term goal is to explore the mechanism by which accessory cells process and present antigen to lymphoid cells and how they function in the regulation of the immune response. Using antigen-specific primed lymph node T cell proliferation and the in vitro primary antibody-forming cell response, the population(s) of cells possessing accessory cell function have been characterized: Thy 1.2 negative, surface Ig negative, I-A and I-E/C subregion antigens positive, radioresistant, glass adherent, phagocytic, Fc receptor positive. Accessory cell function required living cells and the efficiency of such function was directly related to the length of time the cells were exposed to antigen. Response to the antigens TNP-(T,G)-A--L and TNP-(H,G)-A--L was controlled by autosomal dominant genes located in the K or I-A regions of the responder H-2 complex and therefore mapped identically to the Ir genes controlling overall in vitro and in vivo responsiveness to these antigens.